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Window on the chemistry of cracking glass.

Window on the chemisty of cracking glass

If glass were flawless, it would be much stronger than steel. However, most glass is full of tiny cracks that can slowly creek along or suddenly shoot across at shattering speeds. To predict how glass will stand up over time, engineers are studying how it cracs and what factors determine the way cracks grow.

Several years ago, Bruce Bunker and Terry Michalske of Sandia National Laboratories in Albuquerque, N.M., developed models describing what happens to individual molecules when glass cracks. Now, they have gained a better understanding by taking into account the way outside chemicals alter the surface of cracks and influence their growth. They presented their findings in Houston this week at the International Conference on Fracture.

Bunker and Michalske's earlier work focused on the very tip of a crack, where the silicon-oxygen network of simple glasses can tear apart at one-trillionth of an inch per hour, then suddenly accelerate to 50 or 60 miles per hour. The researchers looked at how various corrosive chemicals -- especially water -- speed up the process.

But that model couldn't satisfactorily predict glass behavior over 10 or 20 years, especially in more complex glasses. Such glasses tend to be more vulnearable to corrosion by water and other chemicals. Corrosives add elements to the crack surface and leach out components of the glass, giving rise to a chemically distinct "alteration" layer. To test the role of this alteration layer in corrosion and cracking, Bunker and Michalske exposed different kinds of glass to different chemicals. They measured how fast the alteration layers formed and how fast cracks grew in each case. They also determined how much the alteration layers stressed the glass by measuring how the corrosive agents warped it.

The researchers found they could predict how long glass would hold up by analyzing the structure of the alteration layer. Generally, if the alteration layer is more compact than the surrounding glass, it will contract the crack surface and pull the crack apart. Other alteration layers expand the crack surface, squeezing the crack shut.

Michalske suggests the work will enable engineers to design glass products taking the expected lifetime into account. In addition, it may lead to new glasses with a chemistry that prevents alteration layers or encourages an alteration layer that hinders cracks.
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Author:Flam, Faye
Publication:Science News
Date:Mar 18, 1989
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